1. Field of the Invention
The present invention relates to a TFT device, particularly to a TFT device of a photoelectric converter suitable for use as a photosensor and/or a driver for driving the photosensor.
2. Related Background Art
Photosensors have been used heretofore as a photoelectric converter of an image information processing apparatus such as facsimile apparatus, digital copying machines and image readers.
Specifically, amorphous silicon hydride (hereinafter denominated a-Si:H) thin film transistors have been used recently as photosensors of a high sensitivity image reader which is constructed of elongated line sensors made of one-dimensionally aligned photosensors. Photosensors using thin film transistors are mainly divided into photodiode type and photoconductive type.
In photodiode type photosensors, since a reverse bias voltage is applied across a junction between electrodes, pairs of light-induced electrons and holes are respectively driven to the corresponding electrodes. Thus, only a primary photocurrent flows without injecting carriers from the electrodes.
In contrast, with the above, a photoconductive type photosensor allows electrons or holes to be injected from an electrode so that the density of electrons or holes within a semiconductor region becomes sufficiently high to obtain considerably larger output current (secondary photocurrent) than that of a photodiode type, by applying a voltage between electrodes.
The following prior art is known and have been proposed with respect to a photoelectric converter having photoconductive type photosensors:
(1) A photosensor shown in FIG. 1 (Journal of the Institute of Image Electronics in Japan, Vol. 15, No. 1, 1986 is hereafter referred to as a Prior Art).
In the photosensor shown in FIG. 1, formed on an insulation substrate 1 such as glass or ceramics is a semiconductor layer 2 serving as a photoconductive layer such as CdS.Se or a-Si:H. There are also formed thereon a pair of main electrodes 4 and 4' on doped semiconductor layers 3 and 3' serving as ohmic contacts. In this case, the doped semiconductor layers 3 and 3' are n-type in case that carriers injected into the semiconductor layer 2 from the electrode are electrons, and p-type if carriers are holes.
With the construction as above, as light is applied to the semiconductor layer 2 from the insulation substrate 1 (provided that the insulation substrate 1 is transparent) side or from the main electrode 4, 4' side, electrons or holes contributing to conduction are light-induced within the semiconductor layer 2 between the main electrodes 4 and 4' and the density thereof becomes high.
Accordingly, as shown in the Figure, as a voltage is applied between the main electrodes 4 and 4', a light-induced large secondary current flows as a signal current to allow a large output to be developed across a load resistor (not shown).
(2) A prior art reference using a TFT device shown in FIG. 2 as a photosensor (Japanese Patent Application No. 142986/1986 is hereafter referred to as Prior Art 1).
This Prior Art 1 has been proposed by the present applicant wherein auxiliary electrodes are provided at a sensor portion to stabilize a photocurrent and improve the linearity of dependence of a photocurrent upon illumination.
FIG. 2 is an illustrative view of a basic structure showing a photoconductive type sensor and its drive element of an improved photoelectric converter already proposed by the present applicant. Similar elements to those shown in FIG. 1 are represented by using identical reference numbers.
Referring to FIG. 2, formed on a transparent or opaque insulation substrate 1 are a gate electrode 5 made of a patterned transparent or opaque conductive layer, and an insulation film 6 made of insulation material such as SiOx or SiNx by means of the sputter method or the glow discharge method. Formed on the insulation film 6 are a semiconductor layer 2 made of a-Si:H serving as a photoconductive layer as discussed before, doped semiconductor layers 3 and 3' made of a-Si:H, and main electrodes 4 and 4' (in this case, the main electrode 4 is a drain electrode and the main electrode 4' is a source electrode).
Assuming that electrons are injected carriers, the doped semiconductor layers 3 and 3' are made of n-type a-Si:H.
With the photoconductive type sensor as above, a DC source 7 is connected between the drain electrode 4 and the source electrode 4', and a variable DC source 8 is connected between the source electrode 4' and the gate electrode 5. The variable DC source 8 can also reverse the polarity.
In this example, the sensor portion operates at a gate negative potential (V.sub.G). As shown in FIG. 3A, according to the space charge distribution of the Poisson formula, the energy band in case of a non-doped i-type a-Si:H layer is usually provided with a depletion area by about 1 micron. Namely, the gate side of the i-type semiconductor layer 13 is strongly inverted to p-type.
(3) A Prior art using the TFT shown in FIG. 2 as a photosensor and as a drive element therefor (EP Laid-Open Publication No. 232,083. Hereinafter referred to as Prior Art 2.
The above-described Prior Arts, however, have the following problems and some points require improvement:
(1) Prior Art
The linearity (.gamma.:Ip.alpha.F) of dependence of a photocurrent upon strength of light is poor (i.e., .gamma. becomes less than 1), wherein F represents the illumination of light incident on the region between the main electrodes 4 and 4' from the main electrodes side of the sensor shown in FIG. 1, and Ip represents a photocurrent flowing through the main electrodes 4 and 4'.
(2) Prior Art 1 has also the following points which can undergo some improvements. First, an example of the linearity (.gamma.:Ip.alpha.F) of dependence of a photocurrent upon light amount is shown in FIG. 4, wherein F represents the illumination of light incident to the region between the main electrodes 4 and 4' from the main electrodes side of the sensor shown in FIG. 2, and Ip represents a photocurrent flowing through the main electrodes 4 and 4'.
In FIG. 4, a curve (a) indicates a photocurrent Ip flowing through the photosensor shown in FIG. 2, and a curve (b) indicates a .gamma. value.
As seen from FIG. 4, if the photosensor of Prior Art 1 operates at a negative potential of the gate electrode 5, the linearity of dependence of a photocurrent upon the light amount is improved but the amount of photocurrent is reduced. That feature should undergo some improvements.
It is considered that the reduction of photocurrent is caused by a low gain G=.mu..tau. E/L of a secondary photocurrent due to a short life of light-induced carriers (electrons in this case) on the gate side of the i-type semiconductor layer which has been strongly inverted to p-type. Symbols of the above equation are defined as:
.mu.: electron mobility PA0 .tau.: life of electron PA0 E: electric field PA0 L: distance between electrodes.
To obtain photocurrent sufficient for operating the line sensor, it is necessary to make a film thickness of the semiconductor layer large, generally thicker than 0.5 micron and in some cases 1 to 2 microns. Therefore, not only the time required for forming a film becomes long, but also the contact hole for the gate electrode must be formed deeper, thus leading to potential failure of contacts and inconvenience in the manufacture.
Further, since the energy band of the semiconductor layer is almost uniformly inclined, the surface of the gap portion between the main electrodes 4 and 4' is susceptible to the influence of ions and moisture, and carriers are likely to be trapped at a deep interfacial level present at the interface between the semiconductor layer and the gate insulation film. Accordingly, the conditions of forming a passivation or an interface have been restricted.
(3) Prior Art 2 has also the following points to be improved.
According to Prior art 2 utilizing the TFT device shown in FIG. 2 as a photosensor and as its drive element, there are occasionally a plurality of traps at the vicinity of the interface between the gate insulation film 12 and the semiconductor layer 13 of the TFT device (See FIGS. 3A-3D) so that carriers are trapped gradually. Therefore, there remain several points which require improvement, notably a threshold voltage V.sub.TH which fluctuates, a drain current which changes in time, the TFT device is susceptible to the effects (caused by traps generated by ions or moisture) of the interface, and the stability and reproductiveness of a semiconductor layer at the vicinity of the interface are poor.